I think I understand, but where do you find force?

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Laura0901

Homework Statement


Find the coefficient of kinetic friction that is used to stop a train engine along its track of the trains velocity is 30 m/s when the brakes are applied, and it takes 30 sec for the 10,000 kg train to stop

Homework Equations


Ffric=MN=Mmg
(I think that's the equation to use.

The Attempt at a Solution


Ffric=M(10000kg)(9.8m/s)
Ffric=M(98,000)
What do you use for the force? 30m/s?
 

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Laura0901 said:

Homework Statement


Find the coefficient of kinetic friction that is used to stop a train engine along its track of the trains velocity is 30 m/s when the brakes are applied, and it takes 30 sec for the 10,000 kg train to stop

Homework Equations


Ffric=MN=Mmg
(I think that's the equation to use.

The Attempt at a Solution


Ffric=M(10000kg)(9.8m/s)
Ffric=M(98,000)
What do you use for the force? 30m/s?
Hello Laura0901. Welcome to PF !

What is the acceleration of the train ?
 
Laura0901 said:
It is not said in the problem.
Of course you must figure it out from the given information.
 
And how would you do that? Using the kinematic equations?
 
Why would we need to find the acceleration? I need the friction force.
 
Laura0901 said:
Why would we need to find the acceleration? I need the friction force.
And, just what force is it that is slowing the train?
 
Laura0901 said:
Either gravity or velocity..I think.
The train slows to a stop. That's an acceleration. That requires a force. (Ask Mr. Newton.)

Gravity is a force, but it acts in a vertical, the train's motion and its acceleration are in the horizontal direction. Gravity is not the force which slows the train.

Velocity is not a force.
 
1) Figure out the force in the x-direction that SammyS is talking about, the force that causes the train to stop.
2) Use kinematics to find a
3)Use Fx=max to find the magnitude of the force mentioned in step one
Then you should have enough data to solve Ffric=M(98,000)
 
There aren't any forces in the x direction. I don't think length is a force. I don't know how to find friction.
 
Laura0901 said:
There aren't any forces in the x direction. .
Then the train's going to keep going in the x direction.

By the way: What direction is the friction force ?
 
Laura0901 said:
The Positive direction

Positive in what sense?
Positive x-direction?
or
Positive as in upward?
​
 
Okay... I hope you are asking if fk is 2N because M is not in Newtons. I don't think fk=2N either though...
 
stephen8686 said:
Okay... I hope you are asking if fk is 2N because M is not in Newtons. I don't think fk=2N either though...
Right. It would take very long time to stop.
 
I don't understand how you'd even get a number just because it's a horizontal force?
 
I think I understand friction now thanks for the video.
 
Laura0901 said:
So is air resistance part of it?
Probably not much. The friction force is stopping a train.

Calculate the acceleration as has been suggested previously.
 
Air resistance is a totally different thing. You'll learn that later in physics. Until then, assume there is no air resistance. Kinetic friction is between two objects sliding on each other.
 
I agree a=1 (or -1 depending on your positive x direction).
 
Thank you for your time. If I get this problem wrong on the quiz, I'll get a 75%. I'll take that..